In grove management, though "drip irrigation" techniques do make it practically impossible to drown a tree by totally cutting off its oxygen supply, control of water oversupply by drip irrigation means does present some problems if crop yields are to be consistently maintained. In orchards, the drip line of a tree is closely associated with the development of the tree's root system. Drip lines in general fall under the outer extent of the canopy of the tree and it is from here that the watershed of the tree naturally irrigates. For optimum control, moisture sensors are best located at the drip line and for optimum distribution, emitters are best spaced around the trunk of the tree just inside and outside the drip line. Good practice in grove management with existing irrigation systems has found it necessary to maintain close monitoring of the systems by the grove manager on a systematic basis if crop yields are to be consistently maintained therewith.
It is characteristic in the irrigation of orchards that a time lag occurs between the time that the error in the moisture content is sensed at a control point in the orchard and the time that a correcting dispensation of water arrives from the emitter some distance away. Time lags in irrigation control systems produce dynamic errors in the current moisture content valves and make irrigation management a closely monitored occupation for the grove manager. In citrus grove management, a temporary overshoot or a continuous instability in the supply of water to the grove can lead to disturbances of tree function and the falling off in production of fruit. Reestablishing production of a disturbed tree can take a very long time. With present-day irrigating equipment, disturbances of tree function can only be eliminated by close monitoring of the soil moisture content around the tree by the irrigation manager.
Reduction of tree disturbance without the need for close monitoring can be had by using fully-automatic feed-back control. Fully-automatic controls that have the capability of coping with the time lags associated with orchard irrigation that lead to tree dormancy are in general mechanically and electrically complex and costly in the first instance and expensive to maintain.
Complexity and cost of an irrigation control can be reduced if a fully-automatic, modulated-pulse width rather than an analog control system is used. Analog controls correct water dispensation to the soil by modulating pump valving or pump speed to vary pump output. Modulated-pulse width controls correct water dispensation by changing pump output at constant speed over varying periods of time. Modulated-pulse width controls do, however, tend to accentuate the aforementioned time lag effects more than do their analog counterparts and, for that reason, are economically unacceptable for orchard irrigation applications.
Unmodulated-pulse width feed-back irrigation controls, having dual setpoints which correct for both errors in deficiency and errors of oversupply of water, are the least expensive of control systems suitable for irrigation applications. In unmodulated-pulse width, dual setpoint irrigation systems, the change in water supply to correct a measured soil moisture content is achieved by "on-off" operation of a constant speed pump for set periods of time. A range of moisture contents is set between which operation is deemed satisfactory and water control periods are preselected which are capable of supplying the water needed therefor. The unmodulated-pulse width, dual setpoint control for irrigation application is mechanically and electrically simpler and less costly than the modulated-pulse or analog type but also tends to accentuate the time lag effects unless closely monitored. For this reason, such systems are likewise uneconomical for irrigation applications.
There is thus an established need for a controller for an irrigation system which maintains crop production at top yield at all times without the need for close monitoring by the irrigation manager in on-the-spot correcting for overshoots and instabilities in the supply of water dispensed thereto.